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MPLS — label switching across a provider core
Service Provider Advanced 5 devices 5 checked tasks 100 points Plan lab
Two customer sites either side of a three-router core. OSPF already carries every prefix, so it all pings before you start. Turn on LDP and watch the middle router stop reading IP addresses altogether — push at the edge, swap in the middle, pop at the penultimate hop. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Two customer sites either side of a three-router core. OSPF already carries every prefix, so it all pings before you start. Turn on LDP and watch the middle router stop reading IP addresses altogether — push at the edge, swap in the middle, pop at the penultimate hop.
Your role: an engineer in the provider’s network operations centre. Level: Advanced Done when: all 5 goals show as passed (100 points).
Network status
Devices: 2 PCs, 3 routers, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. CE1 is a host with 10.0.1.10/24, gateway 10.0.1.1. PE1 starts with addresses (L0 1.1.1.1/32, G0/0 10.0.1.1/24, G0/1 192.168.12.1/24) and OSPF 1 already configured. P starts with addresses (L0 2.2.2.2/32, G0/0 192.168.12.2/24, G0/1 192.168.23.2/24) and OSPF 1 already configured. PE2 starts with addresses (L0 3.3.3.3/32, G0/0 192.168.23.3/24, G0/1 10.0.2.1/24) and OSPF 1 already configured. CE2 is a host with 10.0.2.10/24, gateway 10.0.2.1. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
Every core router picks its LDP router-id from its loopback (15 pts) LDP sessions are up across both core links (25 pts) PE1 has allocated a label for the far customer prefix (20 pts) PE1 has a forwarding entry towards 10.0.2.0/24 (20 pts) The two customer sites still reach each other (20 pts) The activity, in order
Every core router picks its LDP router-id from its loopback Why: Each goal is a state the lab can check on the running device. Verify with: show running-config LDP sessions are up across both core links Why: Each goal is a state the lab can check on the running device. Verify with: show running-config PE1 has allocated a label for the far customer prefix Why: Each goal is a state the lab can check on the running device. Verify with: show running-config PE1 has a forwarding entry towards 10.0.2.0/24 Why: Each goal is a state the lab can check on the running device. Verify with: show running-config The two customer sites still reach each other Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.0.2.10 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Why this matters
Two customer sites either side of a three-router core. OSPF already carries every prefix, so it all pings before you start. Turn on LDP and watch the middle router stop reading IP addresses altogether — push at the edge, swap in the middle, pop at the penultimate hop.
What you will be able to do
Every core router picks its LDP router-id from its loopback
LDP sessions are up across both core links
PE1 has allocated a label for the far customer prefix
PE1 has a forwarding entry towards 10.0.2.0/24
The network
PC, ISR 2901 — 5 devices in all, already cabled.
Fa0 Gi0/0 10.0.1.1/24 Gi0/1 192.168.12.1/24 Gi0/0 192.168.12.2/24 Gi0/1 192.168.23.2/24 Gi0/0 192.168.23.3/24 Gi0/1 10.0.2.1/24 Fa0 CE1 10.0.1.10/24 · gw 10.0.1.1 PE1 P PE2 CE2 10.0.2.10/24 · gw 10.0.2.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
CE1 PC 10.0.1.10 / 255.255.255.0 gw 10.0.1.1 PE1 ISR 2901 configure it P ISR 2901 configure it PE2 ISR 2901 configure it CE2 PC 10.0.2.10 / 255.255.255.0 gw 10.0.2.1
Cabling
CE1 Fa0 ↔ PE1 Gi0/0 PE1 Gi0/1 ↔ P Gi0/0 P Gi0/1 ↔ PE2 Gi0/0 PE2 Gi0/1 ↔ CE2 Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
PE1 interface Loopback0
ip address 1.1.1.1 255.255.255.255
exit
interface g0/0
ip address 10.0.1.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 192.168.12.1 255.255.255.0
no shutdown
exit
router ospf 1
router-id 1.1.1.1
network 0.0.0.0 255.255.255.255 area 0 P interface Loopback0
ip address 2.2.2.2 255.255.255.255
exit
interface g0/0
ip address 192.168.12.2 255.255.255.0
no shutdown
exit
interface g0/1
ip address 192.168.23.2 255.255.255.0
no shutdown
exit
router ospf 1
router-id 2.2.2.2
network 0.0.0.0 255.255.255.255 area 0 PE2 interface Loopback0
ip address 3.3.3.3 255.255.255.255
exit
interface g0/0
ip address 192.168.23.3 255.255.255.0
no shutdown
exit
interface g0/1
ip address 10.0.2.1 255.255.255.0
no shutdown
exit
router ospf 1
router-id 3.3.3.3
network 0.0.0.0 255.255.255.255 area 0 Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 Every core router picks its LDP router-id from its loopback 15 Checked automatically against the device’s live state (ldpRid). and Checked automatically against the device’s live state (ldpRid). and Checked automatically against the device’s live state (ldpRid). 2 LDP sessions are up across both core links 25 Checked automatically against the device’s live state (ldpNbrs). 3 PE1 has allocated a label for the far customer prefix 20 Checked automatically against the device’s live state (mplsLabel). 4 PE1 has a forwarding entry towards 10.0.2.0/24 20 Checked automatically against the device’s live state (mplsLfib). 5 The two customer sites still reach each other 20 On CE1: ping 10.0.2.10 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
PE1 interface g0/1
mpls ip P interface g0/0
mpls ip
exit
interface g0/1
mpls ip PE2 interface g0/0
mpls ip Where this is examined
This lab practises these topics in the zNetLaB certification courses:
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